This invention relates to crosslinked poly(amino acids) and improved methods to prepare the crosslinked poly(amino acids). In particular, this invention relates to the use of polyaziridines and polyepoxides as crosslinkers for poly(amino acids) to produce superabsorbent polymers under aqueous conditions without handling hydrogel intermediates.
Superabsorbent polymers are capable of absorbing large quantities of water and aqueous solutions, in particular physiological saline solutions, and find use in a variety of applications including, for example, sanitary goods, hygienic materials, water retaining agents and controlled release vehicles. Superabsorbent polymers are generally water-soluble polymers that have been rendered insoluble by a crosslinking process, resulting in a swellable polymer capable of absorbing many times its original weight in water or aqueous solutions, typically at least 20 times the original weight of the superabsorbent. Superabsorbent polymers are generally polyanionic and hydration of charged moeities in the crosslinked polymer provide the driving force for absorbent properties.
Conventional superabsorbent polymers include, for example, poly(carboxylic acids) such as poly(acrylic acids), hydrolyzed acrylonitrile polymers, poly(vinyl alcohols) and starch-acrylic acid graft polymers. U.S. Pat. Nos. 4,541,871 and 4,645,789 disclose the use of polyepoxide and polyaziridinyl compounds, respectively, to provide water-absorbent polyelectrolytes based on acrylic acid. However, water-absorbent polymers based on acrylic acid backbone chemistry have the disadvantage of not being readily biodegradable and thus contribute a burden to the enviroment when they are disposed of or released into effluent streams.
Attempts to provide biodegradable superabsorbent polymers include the crosslinking of polysuccinimide (hereinafter referred to as PSI) followed by hydrolysis, where the reaction of PSI with the crosslinking agent typically requires a non-aqueous solvent, and hydrolysis of the partially crosslinked PSI to the amino acid derivative involves special handling procedures for the hydrogel intermediates (for example, U.S. Pat. No. 5,525,682). U.S. Pat. No. 5,284,936 discloses the preparation of crosslinked poly(amino acids) by thermal crosslinking of polyaspartate with aspartate and lysine and subsequent hydrolysis to provide superabsorbent polymers.
The present invention seeks to overcome the problems associated with prior art processes used to prepare superabsorbent crosslinked poly(amino acids) that are biodegradable by providing an efficient crosslinking process without the involvement of special handling steps for hydrogel intermediates.